Bump flake8 from 3.7.9 to 3.8.4

Bumps flake8 from 3.7.9 to 3.8.4.

Commits

• fb91b99 Release 3.8.4
• b5b4792 Merge branch 'py34_virtualenv_fix' into 'master'
• d3c55c1 limit the virtualenv version in python3.4
• d0a0cb4 Merge branch 'video' into 'master'
• eb9ca33 add video tutorial to plugin development page
• a42d8cb Merge branch 'spelling' into 'master'
• 2fe70dd fix a few small spelling issues
• 93209ff Merge branch 'fix_linting_end_of_file' into 'master'
• ee9c287 fix skipping of physical checks when file does not end in newline
• 2c64d3e Merge branch 'docs-options-defaults' into 'master'
• Additional commits viewable in compare view